Effects of Hydrology on Fish Diversity and Assemblage Structure in a Texan Coastal Plains River

Abstract

High-flow pulses affect river ecosystem dynamics in many important ways including by forming connections between the channel and oxbow lakes. This study assessed the influence of discharge on fish assemblage structure and diversity in the channel and oxbows of the Guadalupe River, Texas. Local assemblages of two oxbows and two channel sites were surveyed using standardized methods to test four hypotheses: (1) assemblage structure in oxbow lakes differs from those in the river channel, (2) α diversity decreases during extended periods of low discharge, (3) β diversity decreases during high discharge and increases during extended periods of low discharge, and (4) species turnover and assemblage nestedness decline during periods of high discharge. We found evidence to support the first three hypotheses. Unsurprisingly, lotic-adapted fishes were observed more frequently in the river channel, whereas lentic-adapted species generally were more common in the oxbow lakes. Species richness declined during periods of low discharge possibly due to harsher environmental conditions or stronger species interactions (e.g., predation). Discharge was inversely associated with both β diversity and species turnover, suggesting a stronger mass effect during high-flow pulses, and stronger species sorting during low-flow conditions. Contrary to our fourth hypothesis, assemblage nestedness increased during periods of high discharge. Finally, we also found evidence to support the hypothesis that species turnover decreased as discharge declined. The results of this study demonstrate the importance of flow pulses for maintaining fish diversity and assemblage structure in floodplain river systems. With additional research involving more sites and longer time series, it should be feasible to define and identify thresholds for flow regime changes that alter assemblage structure and species diversity.

Spatial Variability in the Fish Assemblage of a Large River–Reservoir Ecosystem

“…Connectivity characteristics (e.g., periodicity and duration) are likely key to supporting the diversity of fish assemblages along lateral habitat gradients in the RRE, similar to previous study results from river floodplain systems (Kwak 1988;Winemiller et al 2000;Miranda 2005;Zeug et al 2005;Bower et al 2019). Multiple unique fish assemblages were identified in Trinity River backwaters with variable connection frequencies to the main river.…”

“…Occurrence of 58 species and alignment of unique assemblages along lateral and longitudinal gradients indicate that this RRE likely maintains many aspects of ecosystem function similar to those that existed prior to dam construction. In general, we found higher levels of species richness in the Trinity River-Lake Livingston ecosystem than were reported for other Texas river systems, including the Brazos River (N = 42-48; Winemiller et al 2000;Zeug et al 2005), Colorado River (N = 41; Buckmeier et al 2014), and Guadalupe River (N = 36; Bower et al 2019). In general, these major Texas river basins would be expected to contain similar richness of freshwater fishes ranging from about 80 to 110 species recorded within each watershed (Hendrickson and Cohen 2015).…”

“…2005; Bower et al. 2019). Multiple unique fish assemblages were identified in Trinity River backwaters with variable connection frequencies to the main river.…”

“…2014), and Guadalupe River ( N = 36; Bower et al. 2019). In general, these major Texas river basins would be expected to contain similar richness of freshwater fishes ranging from about 80 to 110 species recorded within each watershed (Hendrickson and Cohen 2015).…”

Ephemeral habitat supports high fish α‐diversity and β‐diversity during drought in a subtropical semiarid wetland

“…It could also be due to increased effects of species sorting as a result of intermediate levels of dispersal as the water gradually dried up before complete loss of connectivity (Heino et al 2015; Leibold & Chase, 2018; Leibold et al 2004). On the other hand, fish diversity was homogenized across habitats during the wet season due to increased connectivity among habitats, suggesting a strong mass effect in community assembly (Amoros & Bornette, 2002; Bower et al 2019; Leibold & Chase, 2018; Leibold et al 2004; Ng et al 2009; Thomaz et al 2007).…”